A long-term evolution (LTE) communication system of 3rd Generation Partnership Project (3GPP) Rel-8 standard and/or the 3GPP Rel-9 standard has been developed as a successor to the universal mobile telecommunications system (UMTS) in order to further enhance the performance of the UMTS and to satisfy increasing demands of users. A LTE system would include an improved radio interface as well as improved radio network architecture to provide a high data rate, low latency, packet optimization, improved system capacity, and improved coverage over a predecessor.
FIG. 1 illustrates a typical LTE system 100 of which a radio access network (RAN) known as an evolved UTRAN (E-UTRAN) would include one or multiple evolved Node-B (eNB) 101 for communicating with one or more user equipment (UEs) 102. The E-UTRAN may communicate with a core network 103 that includes a mobility management entity (MME) 104, a serving gateway (S-GW) 105, and etc., for Non Access Stratum (NAS) control.
A LTE communication system currently has available at least two division duplexing mechanisms which would include the frequency-division duplexing (FDD) mechanism and the time-division duplexing (TDD) mechanism. When FDD is being implemented, a UE would be able to transmit and receive signals in different carriers simultaneously. When TDD is being implemented, a UE would be able to separate uplink and downlink transmissions in different time slots. Also, a TDD system may offer flexible resource utilizations through different TDD configurations.
FIG. 2 illustrates seven different UL/DL configurations currently implemented by a LTE communication system. Based on the traffic characteristic, different DL:UL ratios ranging from 2:3 to 9:1 could be selected at one time as specified in a FIG. 2. In detail, any “U” (e.g., 201) represents that the subframe is a UL subframe where UL data is transmitted, any “D” (e.g., 202) represents that the subframe is a DL subframe where DL data is transmitted, and any “S” (e.g., 203) represents that the subframe is a special subframe where control information and maybe data (according to the special subframe configuration) is transmitted.
FIG. 3A illustrates a typical downlink subframe 300 of a LTE communication system. The downlink subframe 300 may comprise a physical downlink control channel (PDCCH) 301 and physical downlink shared channel (PDSCH) 303. The PDSCH 303 is a data bearing channel which is allocated for a specific user. The indicate a downlink assignment. When a UE detects a downlink assignment in a subframe, the UE will receive a corresponding PDSCH in the same subframe. In general, a PDSCH typically is scheduled by a downlink assignment in the same subframe as shown in FIG. 3B.
The DCI 302 may indicate a downlink resource in the PDSCH 303 for scheduling a downlink resource of hybrid automatic repeat request (HARQ) process. The DCI 302 may include a field that indicates a DL HARQ process number where the related maximum numbers of DL HARQ processes (MDL_HARQ) are shown in FIG. 4A. As it can be seen in FIG. 4A that the (per carrier) MDL_HARQ would be related to the TDD/FDD duplex setting and/or the TDD configuration operated by a communication device. For example, MDL_HARQ 402 would typically be set to 8 for a FDD carrier 401. The MDL_HARQ 402 would be set as 4, 7, 10, 9, 12, 15 and 6 for the TDD carrier 403 with UL/DL configurations 0, 1, 2, 3, 4, 5 and 6, respectively. Thus, 3 bits in DCI are needed to represent up to 8 HARQ processes for the FDD carrier 401, whereas for the TDD carrier 403, 4 bits in DCI are needed to represent various maximum numbers of HARQ processes according to UL/DL configurations.
FIG. 4B is a flow chart which illustrates the current LTE HARQ retransmission process. In step S411a, a base station would transmit a first transmission in a first subframe via a serving cell. In step S411b, a UE would receive a first transmission in a first subframe via a serving cell. In step S412, the UE would decode the first transmission. In step S413a, a UE would transmit an acknowledgment (ACK) or negative acknowledgment (NACK) in a second subframe wherein the ACK or NACK (ACK/NACK) is corresponding to the decoding result. In step S413b, the base station would receive an acknowledgment (ACK) or negative acknowledgment (NACK) in a second subframe corresponding to the first transmission. In step S414a, a base station would transmit a second transmission in a third subframe via the serving cell wherein the second transmission is a retransmission of the first transmission. In step S414b, the UE would receive a second transmission in a third subframe via the serving cell wherein the second transmission is a retransmission of the first transmission. For the current LTE HARQ retransmission process, when a first transmission is transmitted in a serving cell, the second transmission would also be transmitted in the same serving cell.
FIG. 5A illustrates an example of HARQ processes in FDD mode. In general, up to 8 downlink HARQ processes, or MDL_HARQ=8, is supported in a conventional FDD system. In FIG. 5A, each of the P0˜P7 signify each of the 8 DL HARQ processes. The DL process number is indicated by the field HARQ number in DCI which is a 3 bit field for FDD and a 4 bit field for TDD. Referring to FIG. 5A, in step S501, assuming that a UE receives a first transmission of a DL HARQ process p0 in subframe index 0, the UE transmits an acknowledgement (ACK)/negative-acknowledge (NACK) corresponding to the first transmission of the DL HARQ process p0 as soon as subframe index 4. In step S502, assuming that the UE transmits a NACK corresponding to the first transmission of the DL HARQ process p0 in subframe index 4, then in step S503, the UE may receive a retransmission of the DL HARQ process p0 in subframe index 8 wherein the first transmission and the retransmission is received on the same serving cell.
In FIG. 5A, during the time between the first transmission and the retransmission, UE may receive up to 8 DL HARQ processes (P0˜P7). Therefore, the maximum number of DL HARQ process is 8 (MDL_HARQ=8) in a conventional FDD system.
FIG. 5B and FIG. 5C illustrates examples of a HARQ processes in TDD mode UL/DL configuration 0 and in TDD mode UL/DL configuration 5 respectively. The MDL_HARQ is related to its UL/DL configuration. In FIG. 5B, the P0˜P3 number signifies each DL HARQ process as the maximum number of DL HARQ processes is 4 in TDD mode UL/DL configuration 0; whereas in FIG. 5C, the P0˜P14 number indicates that the maximum number of DL HARQ processes is 15 in TDD UL/DL configuration 5. In step S511, assuming that a UE receives a first transmission of a DL HARQ process p0 in subframe index 0 of the radio frame m. In step S512, the UE transmits a NACK corresponding to the first transmission of the DL HARQ process p0 in a subframe configured for uplink as soon as subframe index 4. However, there is no downlink subframe configured with 4 subframes of subframe index 4, and therefore in step S513, the UE receives a retransmission of the DL HARQ process p0 in subframe index 0 of the frame m+1 wherein the first transmission and the retransmission is received on the same serving cell.
In FIG. 5B, during the time between the first transmission and the retransmission, UE may receive up to 4 DL HARQ processes (P0˜P3). Therefore, the maximum number of DL HARQ process is 4 (MDL_HARQ=4) in a conventional TDD system if the UL/DL configuration is 0.
AS for the example shown in figure SC, in step S521, assuming that a UE receives a first transmission of a DL HARQ process p0 in subframe index 0 of the frame m. However, there is no uplink subframe available for uplink within 4 subframes of subframe index 0, and therefore, in step S522, the UE transmits a NACK corresponding to the first transmission of the DL HARQ process p0 in a next available uplink subframe which is subframe index 2 of the frame m+1. In step S523, the UE receives a retransmission of the DL HARQ process p0 in subframe index 6 of the frame m+1 wherein the first transmission and the retransmission are received on the same serving cell.
In FIG. 5C, during the time between the first transmission and the retransmission, UE may receive up to 15 DL HARQ processes (P0˜P14). Therefore, the maximum number of DL HARQ process is 15 (MDL_HARQ=15) in a conventional TDD system if the UL/DL configuration is 5.
A LTE-advanced (LTE-A) system, as its name implies, is an upgrade over Rel-8 and Rel-9 of the LTE system. A LTE-A system targets faster switching between power states, improves performance at the coverage edge of an eNB, and includes advanced techniques, such as carrier aggregation (CA), coordinated multipoint (CoMP) transmissions/reception, uplink (UL) multiple-input multiple-output (MIMO), etc. For a UE and an eNB to communicate with each other in a LTE-A system, the UE and the eNB may adhere to standards developed for the LTE-A system, such as the 3GPP Rel-10 standard or later versions.
CA is introduced to the LTE-A Rel-10 system and beyond by which more than one carriers (e.g., component carriers, serving cells) may be aggregated to achieve a wider band transmission. The CA would increase bandwidth flexibility by aggregating multiple carriers. When a UE is configured with the CA, the UE has the ability to receive and/or transmit packets via one or multiple carriers to increase the overall system throughput. FIG. 6A illustrates an example of a FDD CA scheme in which 2 FDD DL serving cells are aggregated, and FIG. 6B illustrates a TDD CA scheme in which 2 TDD serving cells are aggregated. Accordingly, the LTE-A system could support a wider bandwidth up to 100 MHz by aggregating a maximum number of 5 serving cells as a maximum bandwidth of 20 MHz is available for each serving cell which is backward compatible with the 3GPP Rel-8 standard. The LTE-A system supports the CA for both contiguous and non-contiguous serving cells in which each serving cell is limited to a maximum of 110 resource blocks. The CA would thus increase bandwidth flexibility by aggregating the serving cells.
When a UE is configured with the CA, the UE has the ability to receive and/or transmit packets via one or multiple serving cells, or namely configured component carriers (CCs), to increase its transmission throughput. If the FDD mode is being implemented in a LTE-A system, it is possible for an eNB to configure a UE for different numbers of uplink (UL) and downlink (DL) serving cells. Otherwise if the TDD mode is being implemented in the LTE-A system, it is possible for an eNB to configure the UE with different TDD UL/DL configurations for different serving cells.
Moreover, serving cells configured to a UE in the FDD mode would typically include one or only one DL primary serving cell (DL PCell) and one or only one UL primary serving cell (UL PCell). As for operating in the TDD mode, the serving cells configured to a UE would typically include one or only one PCell and one or more secondary serving cell (SCell). The number of the configured SCell is arbitrary and would typically be related to UL and/or DL aggregation capabilities of the UE and available radio resources.
The hybrid automatic repeat request (HARQ) process has been used in a LTE system to provide efficient and reliable communications. As being different from an automatic repeat request (ARQ) process, a forward correcting code (FEC) has been used for a HARQ process. For example, a mobile device may feedback a positive acknowledgment (ACK) to inform a network that a packet has been received correctly by the mobile device assuming that the mobile device has decoded the packet correctly. Oppositely, the mobile device may feedback a negative acknowledgment (NACK) to the network if the mobile device cannot decode the packet correctly. Under the circumstance in which NACK has been received by a UE, the UE may store a part or the whole of a received data packet in a soft buffer of the UE.
A soft buffer size of a UE could be indicated by the total number of soft channel bits (Nsoft) and is a function of its UE category as illustrated in FIG. 7. The detailed descriptions with regard to specific operations related to FIG. 7 are described in 3GPP TS 36.306, “E-UTRA; User Equipment (UE) radio access capabilities (Release 12)” which is incorporated by reference. In response to receiving a retransmitted packet from a wireless transmitter of another wireless node, the UE may combine the soft values of the retransmitted packet and the stored packet. The receiver of the UE may decode the packets by using the combined soft values. Furthermore, the combination of a one or more former erroneously received packets and a currently received packet would increase the probability of a successful decode. The UE would continue the HARQ process until a packet is decoded correctly or until a maximum number of retransmissions have been sent. When the maximum number of retransmissions has been exhausted, the HARQ process would produce a failure and the consequently the UE may allow the ARQ process of a radio link control (RLC) to try again. In other words, the space of a soft buffer would be reserved for the HARQ process such that the UE could store results of a HARQ process which has not been decoded correctly. Otherwise, if a soft buffer is fully occupied, the HARQ could potentially be hindered. When multiple packets are transmitted to the UE, the UE would normally need to store multiple HARQ processes because of unsuccessful attempts of decoding packets.
As stated previously, a UE could store a maximum of 8 HARQ processes per serving cell in a soft buffer in a LTE/LTE-A system. Each HARQ process may carry at least one packet. A packet could be, for example, a transport block in a LTE system. A transport block (TB) is a data unit transmitted on a PDSCH (e.g., 303) from an eNB (e.g., 101) to at least one UE (e.g., 102) in a LTE radio subframe. Each LTE radio subframe has the duration of 1 millisecond (ms). Each LTE radio frame is 10 ms and contains 10 LTE radio subframes. When operating under a method of multiple input multiple output (MIMO) such as spatial multiplexing for example, more than one transport blocks could be transmitted per transmission time interval (TTI) for a UE. Thus, a soft buffer partition method in a network comprising at least one serving cell is introduced as follows.
The following descriptions are made with reference to FIG. 4A and FIG. 7. According to 3GPP TS 36.213, “E-UTRA; Physical layer procedures (Release 12)” which is incorporated by reference, Nsoft could be divided into multiple partitions for storing soft channel bits according to:
                              n                      SB            ⁢                                                                =                  min          ⁡                      (                                          N                cb                            ,                              ⌊                                                      N                    soft                                                        C                    ·                                          N                      cells                      DL                                        ·                                          K                      MIMO                                        ·                                          min                      ⁡                                              (                                                                              M                                                          DL                              ⁢                              _                              ⁢                              HAR                              ⁢                              Q                                                                                ,                                                      M                            limit                                                                          )                                                                                            ⌋                                      )                                              (                  Equation          ⁢                                          ⁢          1                )            To describe more plainly, for FDD, TDD and FDD-TDD, if a UE is configured with more than one serving cell or if a UE is configured with a secondary cell group (SCG), then for each serving cell, for at least (KMIMO·min(MDL_HARQ,Mlimit)) transport blocks, upon detecting a decoding failure of a code block of a transport block, the UE may store received soft channel bits corresponding to a range of at least nSB soft channel bits, where:C is the number of code blocks of the transport block (TB).Ncb is the size of code block of the transport block (TB).KMIMO is the maximum number of transport blocks transmittable to the UE in the TTI of the serving cell.Mlimit is a positive value which equals to 8.MDL_HARQ is the maximum number of DL HARQ processes as shown in FIG. 4.NcellsDL is the number of configured serving cells across both mandatory cell group(MCG) and secondary cell group (SCG) if the UE is configured with a SCG; otherwise,NcellsDL is the number of configured serving cells.min(MDL_HARQ,Mlimit) compares MDL_HARQ and Mlimit and returns the smaller one of MDL-HARQ and Mlimit.
FIG. 8 illustrates the steps of a conventional method of soft buffer partitioning. As shown in Equation 1, the soft buffer is partitioned for each configured serving cell and/or each cell group while the UE is configured with more than one serving cell or if the UE is configured with a secondary cell group (SCG). The soft buffer is partitioned according to the following steps:
In step S801, the UE would determine the total number of soft channel bits (Nsoft) and the number of DL serving cells (NcellsDL).
In step S802, the UE may divide the entire soft buffer into NcellsDL sub-blocks of soft buffer for each serving cell if the UE is configured NcellsDL serving cells wherein each sub-block of soft buffer with a size
      ⌊                  N        soft                    N        cells        DL              ⌋    .
In step S803, the UE may further divide each sub-block of soft buffer into min(MDL_HARQ,Mlimit) partitions for a HARQ process wherein each partition for a HARQ process with a size
      ⌊                  N        soft                              N          cells          DL                ·                  min          ⁡                      (                                          M                                  DL                  ⁢                  _                  ⁢                  HARQ                                            ,                              M                limit                                      )                                ⌋    .
For example, if a UE is configured NcellsDL serving cells, the soft buffer is partitioned for the NcellsDL serving cells. In other words, the entire soft buffer may be divided into NcellsDL sub-blocks for each serving cell if the UE is configured NcellsDL serving cells where each sub-block of soft buffer has a size of
      ⌊                  N        soft                    N        cells        DL              ⌋    .For each serving cell, up to min(MDL_HARQ, Mlimit) HARQ processes could be stored in the soft buffer, and the soft buffer size for each HARQ process is at least
  ⌊            N      soft                      N        cells        DL            ·              min        ⁡                  (                                    M                              DL                ⁢                _                ⁢                HARQ                                      ,                          M              limit                                )                      ⌋soft channel bits. Furthermore, for each transport block within the HARQ process, the soft buffer size for each HARQ process is at least
  ⌊            N      soft                      N        cells        DL            ·              K        MIMO            ·              min        ⁡                  (                                    M                              DL                ⁢                _                ⁢                HARQ                                      ,                          M              limit                                )                      ⌋soft channel bits.
An application of the concept of FIG. 8 is shown in FIG. 9 which illustrates the steps of a conventional soft buffer partitioning in a FDD system with 3 DL serving cells (one DL PCell and two DL SCells), and thus NcellsDL is equal to 3. In this FDD example, MDL_HARQ is equals to 8 for each DL serving cell, and transmit diversity is configured to the UE; hence, KMIMO is set to be 1. Referring FIG. 8 and FIG. 9 together, the soft buffer is partitioned as follows. In step S801, the UE would determine the total number of soft channel bits (Nsoft) (e.g., 905) and the number of DL serving cells (e.g., NcellsDL=3). In step S802, the UE may divide the entire soft buffer into 3 sub-blocks (e.g., 902, 903, 904) of soft buffer for each serving cell if the UE is configured 3 serving cells wherein each sub-block of soft buffer with a size
      ⌊                  N        soft            3        ⌋    .In step S803, the UE may further divide each sub-block of soft buffer into 8 partitions for a HARQ process wherein each partition (e.g., 901) for a HARQ process with a size
      ⌊                  N        soft            24        ⌋    .
Referring to FIG. 9, the UE would divides the entire soft buffer into 3 sub-blocks of soft buffer including 1st sub-block 902, 2nd sub-block 903, and 3rd sub-block 904 for each serving cell if the UE is configured with 3 serving cells. Each sub-block of soft buffer is divided into 8 partitions (e.g., 1-1˜1-8 for the 1st sub-block 902, 2-1˜2-8 for the 2nd sub-block 903, 3-1˜3-8 for the 3rd sub-block 904) for a HARQ process wherein each partition for a HARQ process with a size
      ⌊                  N        soft            24        ⌋    .The 1st sub-block 902 could be for a PCell, the 2nd sub-block 903 could be for a first SCell, and the 3rd sub-block 904 could be for a second SCell.
As an application of FIG. 8, FIG. 10 illustrates the steps of a conventional soft buffer partitioning in a TDD system with 3 TDD serving cells. In this example, the 3 TDD serving cells would include a PCell with UL/DL configuration 0 and two SCells with UL/DL configuration 5, and thus NcellsDL is equal to 3. In this example, MDL_HARQ is equals to 4 and 15 for the PCell and SCells respectively. And transmit diversity is configured to the UE; hence, KMIMO is set to be 1. Referring to FIG. 8 and FIG. 10 together, the soft buffer is partitioned as follows. In step S801, the UE would determine the total number of soft channel bits (Nsoft) (e.g., 1006) and the number of DL serving cells (e.g., NcellsDL=3). In step S802, the UE may divide the entire soft buffer into 3 sub-blocks (e.g., 1003, 1004, 1005) of soft buffer for each serving cell if the UE is configured 3 serving cells wherein each sub-block of soft buffer with a size
      ⌊                  N        soft            3        ⌋    .In step S803, the UE may further divide each sub-block of soft buffer into min(MDL_HARQ,Mlimit) partitions (e.g., 1001, 1002) for a HARQ process wherein each partition for a HARQ process with a size
      ⌊                  N        soft                    3        ·                  min          ⁡                      (                                          M                                  DL                  ⁢                  _                  ⁢                  HARQ                                            ,                              M                limit                                      )                                ⌋    .For PCell, each partition (e.g., 1001) for a HARQ process has a size
      ⌊                  N        soft            12        ⌋    .For SCell, each partition (e.g., 1002) for a HARQ process has a size
      ⌊                  N        soft            24        ⌋    .
Referring to FIG. 10, the entire soft buffer 1006 is divided into 3 sub-blocks of soft buffer (1st sub-block 1003, 2nd sub-block 1004, and 3rd sub-block 1005) for each serving cell if the UE is configured 3 serving cells. For PCell, the sub-block of soft buffer is divided into 4 partitions (1-1˜1-4 for the 1st sub-block 1003 for a HARQ process wherein each partition for a HARQ process with a size
      ⌊                  N        soft            12        ⌋    .For SCell, the sub-block of soft buffer is divided into 8 partitions (2-1˜2-8 for the 2nd sub-block 1004, 3-1˜3-8 for the 3rd sub-block 1005) for a HARQ process wherein each partition for a HARQ process with a size
      ⌊                  N        soft            24        ⌋    .The 1st sub-block 1003 would be for the PCell, the 2nd sub-block 1004 would be for the first SCell, and the 3rd sub-block 1005 would be for the second SCell.
FIG. 11 illustrates a conventional DL HARQ ACK or NACK (ACK/NACK) feedback timeline in a FDD system. For a FDD system, a UE transmits a HARQ ACK/NACK feedback in response to at least one PDSCH transmission in subframe n to report a DL HARQ transmission indicated by a corresponding DL control channel (e.g., physical downlink control channel (PDCCH) or enhanced physical downlink control channel (ePDCCH)) within subframe n−4. As shown in FIG. 11, the ACK/NACK response would be transmitted within 4 subframes of an initial transmission.
As for a TDD single serving cell system, the downlink association set index K: {k0, k1, . . . , kM-1} for TDD of FIG. 12 would apply wherein M is the number of elements in the downlink association set, and the downlink association set comprising at least one element. The detailed descriptions related to the application of FIG. 12 could be seen in Table 10.1.3.1-1 of TS 36.213 which is incorporated by reference. Essentially, a UE would transmit a HARQ ACK or NACK (ACK/NACK) feedback in response to at least one PDSCH transmission in subframe n 1202 to report a DL HARQ transmission as indicated by a corresponding DL control channel (e.g., physical downlink control channel (PDCCH) or enhanced physical downlink control channel (ePDCCH)) within subframe n−k where kεK related to its UL/DL configuration 1201 of the serving cell. In plain language, for configuration 0 as an example, a UE would transmit a DL HARQ ACK/NACK feedback in subframe index 1202 n=4 in response to receiving a transmission in subframe index 0 (4−4=0). Similarly, a UE would transmit a HARQ ACK/NACK feedback in subframe index 7 and 9 in response to receiving a transmission in subframe index 1 (i.e., 7−6=1) and subframe index 5 (i.e., 9−4=5) respectively.
FIG. 13A illustrates DL HARQ ACK or NACK (ACK/NACK) feedback timeline in a TDD single serving cell system which is configured with UL/DL configuration 0. Referring to both FIG. 12 and FIG. 13A, a UE would transmit a DL HARQ ACK/NACK feedback in response to at least one physical DL shared channel (PDSCH) transmission in subframe n to report DL HARQ transmission indicated by a corresponding DL control channel (e.g., physical downlink control channel (PDCCH) or enhanced physical downlink control channel (ePDCCH)) within subframe n−k where kεK related to its UL/DL configuration 0. In this example, the UE would transmit a DL HARQ ACK/NACK feedback in subframe 4, 7, or 9 of frame m in order to report DL HARQ transmission which is indicated by a corresponding DL control channel (e.g., physical downlink control channel (PDCCH) or enhanced physical downlink control channel (ePDCCH)) within subframe 0, 1, or 5 of frame m. Subsequently the UE would transmit DL HARQ ACK/NACK feedback in subframe 2 of frame m+1 in order to report DL HARQ transmission indicated by a corresponding DL control channel (e.g., physical downlink control channel (PDCCH) or enhanced physical downlink control channel (ePDCCH)) within subframe 6 of frame m.
In the example as illustrated by FIG. 13, the transmission of a HARQ ACK/NACK would be delayed because of the unavailability of an uplink subframe within 4 subframes. The special subframe “S” would be considered as a downlink subframe. Similar concept would be applicable for DL HARQ ACK/NACK feedback timeline in a TDD single serving cell system which is configured with UL/DL configuration 1 as illustrated in FIG. 13B.
FIG. 13C illustrates DL HARQ ACK or NACK (ACK/NACK) feedback timeline in a TDD single serving cell system which is configured with UL/DL configuration 5. For FIG. 13C, a UE would transmit a DL HARQ ACK/NACK feedback in response to at least one PDSCH transmission in subframe n in order to report DL HARQ transmission indicated by a corresponding DL control channel (e.g., physical downlink control channel (PDCCH) or enhanced physical downlink control channel (ePDCCH)) within subframe n−k where kεK related to its UL/DL configuration 5. In this example, UE should transmit DL HARQ ACK/NACK feedback in subframe 2 of frame m+2 to report DL HARQ transmission indicated by a corresponding DL control channel (e.g., physical downlink control channel (PDCCH) or enhanced physical downlink control channel (ePDCCH)) within subframe 9 of frame m and DL HARQ transmission indicated by a corresponding DL control channel (e.g., physical downlink control channel (PDCCH) or enhanced physical downlink control channel (ePDCCH)) within subframe 0/1/3/4/5/6/7/8 of frame m+1.
For TDD inter-band CA case, at least one SCell may be configured with a different UL/DL configuration from the PCell. In this particular situation, for a serving cell c, a UE may transmit a DL HARQ ACK or NACK (ACK/NACK) feedback in response to at least one physical DL shared channel (PDSCH) transmission in subframe n in order to report DL HARQ transmission indicated by a corresponding DL control channel (e.g., physical downlink control channel (PDCCH) or enhanced physical downlink control channel (ePDCCH)) within subframe n−k where kεKC related to its DL-reference UL/DL configuration of the serving cell wherein KC is the downlink association set of the serving cell c. Moreover, the determination of KC according to its DL-reference UL/DL configuration is described as follows.
For PCell, DL-reference UL/DL configuration is PCell's UL/DL configuration. For SCell, DL-reference UL/DL configuration is determined according to FIG. 14. FIG. 14 illustrates different sets 1401 of DL-reference UL/DL configurations 1403 for serving cell based on one or more pairs formed by primary cell UL/DL configuration and secondary cell UL/DL configuration 1402.
As an example, FIG. 15 illustrates DL HARQ ACK or NACK (ACK/NACK) feedback timeline in a TDD CA system in which the PCell 1501 is configured with UL/DL configuration 0 and SCell 1502 is configured with UL/DL configuration 5. Thus, DL-reference UL/DL configuration of PCell 1501 is UL/DL configuration 0 and DL-reference configuration of SCell 1502 is UL/DL configuration 5, as shown in FIG. 15. Since the DL HARQ ACK/NACK feedback timeline is determined according to the serving cell's DL-reference UL/DL configuration, DL HARQ ACK/NACK feedback timeline of PCell and SCell is UL/DL configuration 0 and 5 respectively. Because different UL/DL configurations are configured to different serving cells, the DL HARQ ACK/NACK feedback corresponding to the same subframe may be transmitted on different subframes according to the corresponding DL-reference UL/DL configurations. In the scenario of FIG. 15, since a UE may receive DL HARQ process in subframe 0 of frame m of both serving cells, the UE may transmit the corresponding DL HARQ ACK/NACK feedback of PCell's DL HARQ process on subframe 4 of frame m, and the UE may transmit the corresponding DL HARQ ACK/NACK feedback of SCell's DL HARQ process on subframe 2 of frame m+1.